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Energies 2017, 10, 413 13 of 26 behaviour of the heat exchangers to design a proper control system. Desideri et al. [74] compared the moving boundary and finite volume techniques in the design of heat exchangers for ORC applications. Zhang et al. [75] studied a multi-variable control strategy for a 100-kW ORC system. Some works consider a “black box” analysis, in which the ORC system is studied at the process level. Maizza and Maizza [76] considered fixed values for the efficiency of the components. Sciubba et al. [77] used a black box analysis to simulate a dual loop ORC for marine applications. This implies that the thermodynamic parameters selected might affect negatively the design of the components. For this reason, some authors have proposed methods in which the selection of the thermodynamic parameters is bound by constraints that arise from practical limitations in the component design [78,79]. Furthermore, some works considered thermo-economic optimization models [80,81]. In such models, researchers identify which components are more responsible for the high specific cost of small-scale ORC systems. Only a few papers deal with fluid selection procedures based on simultaneous optimization of thermodynamic performance and component design. Quoilin et al. [82] compared screening methods and operating map methods as the most common ones considered in the literature. They state that screening methods are the most commonly used in the literature and that they can be misleading in the process of fluid selection. The operating map methods deal with the interaction between working fluid and expander type. He considered a radial inflow turbine, a screw and a scroll expander. Franchetti et al. [83] considered the operating map method to select the proper working fluid for an ORC in which a radial inflow turbine is selected as the expander. Based on the literature survey, the authors provide general guidelines to underline those properties of the working fluid that have an impact on the specific cost of small-scale ORCs: • Price: Since organic fluids are expensive (20–30 e/kg), it is important to find the correct trade-off between cost and performance [71]. Generally, fluids used in operating ORC plants are extensively used in other fields, which lowers their price. • Density: Chen et al. [84] underline that low density leads to a high volumetric flow rate of the working fluid. This has an impact on different components of the system. The higher the volumetric flow rate, the bigger the size of the components and, in turn, their cost. However, a high volumetric flow rate allows for the reduction of the rotational speed of the expander, which has a positive impact on the reliability of turbo-expanders. • Condensation pressure: Ideally, the condensation pressure should be as close as possible to atmospheric. In fact, a high condensing pressure leads to an increase in the overall system pressure, which requires more resistant and therefore more expensive materials. Condensing pressures below 0.5 bar leads to an increase in the sealing costs to prevent air from entering the system. Moreover, lower pressures increase the size of the condenser. [14]. • Freezing point: The freezing point needs to be well below the minimum ambient temperature in the site in which the system is installed, to avoid solidification of the working fluid during periods of inactivity of the ORC plant [85]. • Cycle top pressure: It is important to keep the cycle top pressure well below the fluid critical pressure [86]. This is necessary to prevent the formation of liquid droplets during the expansion process and to overcome instability during vaporization. Low pressure allows for the use of less expensive materials, which has a positive impact on system costs. High pressure implies high fluid density, which in turns lowers the system size. A trade-off has to be found through a techno-economic analysis. • Heat transfer coefficient: The heat transfer coefficient plays a crucial role in the definition of the size and cost of the heat exchangers [87]. The selection of an organic compound with good heat transfer properties would lead to the reduction of the heat transfer surface, which, in turn, lowers the overall size, weight and cost of the ORC system [88].PDF Image | Small Scale Organic Rankine Cycle (ORC)
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